A vertical transistor in which the gate electrode is embedded in a trench is used in order to achieve miniaturization and high performance of a transistor. In the vertical transistor, a drain-to-source breakdown voltage (hereinafter, simply referred to as “breakdown voltage”) and on-state resistance have a tradeoff relationship. That is, when the impurity concentration of the drift layer is increased so as to decrease the on-state resistance, the breakdown voltage is lowered. To the contrary, when the impurity concentration of the drift layer is decreased so as to improve the breakdown voltage, the on-state resistance is increased.
As a method of improving the tradeoff relationship between the breakdown voltage and the on-state resistance, a field plate electrode may be provided in the trench below the gate electrode. When the electric field distribution in the drift layer is changed by the presence of the field plate electrode, the breakdown voltage of the vertical transistor is improved in comparison to no field plate electrode when the impurity concentration of the drift layer is maintained the same.
In addition to the improvement of the on-state resistance and the breakdown voltage, it is desirable to decrease parasitic capacitance of the device. For example, in order to improve the switching efficiency of a transistor, it is desirable to decrease an output capacitance (Coss) and an output charge amount (Qoss) at the time of charging and discharging the output capacitance. In addition, in order to prevent switching noise of the transistor, it is desirable to optimize the change (hereinafter, referred to as Coss-Vds curve) of the output capacitance (Coss) in accordance with the change of a drain-to-source voltage (Vds).